Examples of a wireless power transmission technique include a wireless power transmission technique of a magnetic resonance method. In the wireless power transmission technique of the magnetic resonance method, a degree of freedom of arrangement of apparatuses is improved since transmission in a range from several centimeters to 10 centimeters is enabled depending on antennas. Furthermore, when the magnetic resonance method is employed, a single power transmission apparatus may transmit electric power to a plurality of power reception apparatuses. In the magnetic resonance method, electric power is supplied by coupling a resonance circuit included in a power transmission apparatus and a resonance circuit included in a power reception apparatus by resonance of a magnetic field. In order to enhance power transmission efficiency of the magnetic resonance method, appropriate impedance of apparatuses should be obtained. In PTL 1, impedance of a matching circuit (resonance circuit) is controlled in accordance with a distance between a power transmission antenna and a power reception antenna. In PTL 2, an impedance control method of a matching circuit is changed on a power transmission apparatus side in accordance with a determination as to whether a power reception apparatus is capable of performing the impedance control of the matching circuit. In PTL 3, a power transmission apparatus and a power reception apparatus are synchronized with each other and resonance frequencies and impedance are changed so that electric power received by a third party is reduced.
Furthermore, in NPL 1, three states including a start phase, an authentication phase, and a power reception (power transmission) phase are defined in a one-to-one wireless power transmission between apparatuses employing an electromagnetic induction method using an electromagnetic induction phenomenon. In the start phase, power is not transmitted and received between the power transmission apparatus and the power reception apparatus. In the authentication phase, the power transmission apparatus transmits electric power which is smaller than transmission electric power in the power reception (power transmission) phase which will be described below to the power reception apparatus. When each of the power transmission apparatus and the power reception apparatus detects the other of the apparatuses, the authentication phase is entered. In the authentication phase, the power reception apparatus activates a communication unit by supplying the electric power supplied from the power transmission apparatus to the communication unit and performs communication for device authentication with the power transmission apparatus through the communication unit. In the authentication phase, power consumption of the power reception apparatus mainly corresponds to power consumption of the communication unit. When authentication is successfully performed in the authentication phase, the power transmission apparatus and the power reception apparatus enter the power reception (power transmission) phase. In the power reception (power transmission) phase, the power transmission apparatus transmits electric power which is larger than the transmission power in the authentication phase to the power reception apparatus. In the power reception (power transmission) phase, the power reception apparatus supplies the electric power supplied from the power transmission apparatus to a load, and the received power is used for charging a battery, for example. In the power reception (power transmission) phase, the power consumption of the power reception apparatus mainly corresponds to power consumption of the load.
In such one-to-one wireless power transmission between apparatuses, in general, impedance is set so that optimum transmission efficiency is attained in the power reception (power transmission) phase taking a circuit size into consideration, and impedance control for optimizing transmission efficiency in all the phases is not performed.